U.S. patent number 5,066,616 [Application Number 07/581,248] was granted by the patent office on 1991-11-19 for method for improving photoresist on wafers by applying fluid layer of liquid solvent.
This patent grant is currently assigned to Hewlett-Packard Company. Invention is credited to William G. Gordon.
United States Patent |
5,066,616 |
Gordon |
November 19, 1991 |
Method for improving photoresist on wafers by applying fluid layer
of liquid solvent
Abstract
A method for applying photoresist to a top surface of a
semiconductor wafer for defining an electronic circuit pattern. The
wafer is placed on a horizontal turntable and liquid solvent is
dispensed onto the wafer's top surface. Spinning the wafer
distributes the solvent to a substantially uniform film thickness
over the entire top surface. Liquid photoresist is dispensed onto
the top surface over the solvent film, preferably while spinning
the wafer, to distribute a photoresist layer over the entire top
surface. Photoresist discharge is controlled so that the wafer
sirface remains entirely wetted by the solvent film during
distribution of the liquid photoresist. The solvent viscosity is
lower than the liquid photoresist viscosity and the solvent film
thickness is sufficient to enable the photoresist to fully cover
any bare silicon, high density or undercut circuit features,
generally in a range of 500 to 10,000 Angstroms and preferably
1,000 to 5,000 Angstroms.
Inventors: |
Gordon; William G. (Corvallis,
OR) |
Assignee: |
Hewlett-Packard Company (Palo
Alto, CA)
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Family
ID: |
27003128 |
Appl.
No.: |
07/581,248 |
Filed: |
September 7, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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365870 |
Jun 14, 1989 |
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Current U.S.
Class: |
430/272.1;
148/DIG.137; 426/96; 426/240; 438/782; 438/948; 427/240;
427/299 |
Current CPC
Class: |
G03F
7/162 (20130101); Y10S 148/137 (20130101); Y10S
438/948 (20130101) |
Current International
Class: |
G03F
7/16 (20060101); H01L 021/00 (); H01L 021/02 ();
H01L 021/30 (); B05D 001/40 () |
Field of
Search: |
;437/225,228,229,231
;148/DIG.137 ;118/52,55,320 ;427/96,99,240
;430/5,272,298,312,327,394 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2743011 |
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Mar 1979 |
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DE |
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0085524 |
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May 1985 |
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JP |
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0081625 |
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Apr 1986 |
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JP |
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0091655 |
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May 1986 |
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JP |
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0150332 |
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Jul 1986 |
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JP |
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0058375 |
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Mar 1989 |
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JP |
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Other References
Moreau, W., Coating Solvent for Resist Films, IBM Tech. Dis. Bull.
(USA), vol. 23, No. 3, p. 991, Aug. 1980. .
Holihan, J., Controlled Gap Photoresist Spinning Process, IBM Tech.
Dis. Bull. (USA), vol. 17, No. 11, p. 3281, Apr. 1975. .
Wolf, S., Silicon Processing for the VLSI Era, vol. 1, pp. 430-434,
Lattice Press, 1986..
|
Primary Examiner: Hearn; Brian E.
Assistant Examiner: Everhart; B.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of application Ser. No.
07/365,870, filed June 14, 1989, now abandoned.
Claims
We claim:
1. In an integrated circuit fabrication process, a method for
uniformly applying photoresist to a top surface of a wafer, the
method comprising:
spreading a liquid solvent substantially uniformly over the wafer
surface to form a liquid solvent film;
applying a liquid photoresist to the wafer surface; and
spreading the photoresist over the solvent film while a thickness
of at least 100 Angstroms of liquid solvent remains on the entire
wafer surface to wet the wafer surface with a mobile layer of fluid
solvent such that the photoresist is distributed in a substantially
uniform layer over the entire surface of the wafer.
2. A method according to claim 1 including spinning the wafer to
spread the liquid solvent thereon and continuing to spin the wafer
during discharge of the photoresist.
3. A method according to claim 2 in which the liquid photoresist is
applied, while the liquid solvent film has a thickness sufficient
to display interference colors under visible light.
4. A method according to claim 2 in which the liquid solvent is
applied while the wafer is static and allowed to stand for a first
time interval, the wafer is next spun at a first rate of rotation
for a second time interval and then spun at a reduced, second rate
of rotation for a third time interval, and the liquid photoresist
discharge is commenced at the end of the third time interval while
the wafer continues to rotate at said second rate of rotation.
5. A method according to claim 4 in which discharge of the liquid
photoresist is stopped after a fourth time interval following
commencement thereof and the wafer is spun at an increased, third
rate of rotation for a fifth time interval.
6. A method according to claim 5 in which the first and second
rates of rotation and the second and third time intervals are
arranged so that the top surface of the wafer remains entirely
wetted by the solvent film during discharge and spreading of the
liquid photoresist.
7. A method according to claim 1 in which the solvent and
photoresist are miscible.
8. A method according to claim 7 in which the liquid solvent and
the liquid photoresist both include EGMEA.
9. A method according to claim 8 in which the liquid solvent
further includes HMDS.
10. A method according to claim 1 in which the solvent has a
viscosity less than a viscosity of the photoresist.
11. A method according to claim 1 in which the solvent film has a
thickness in a range of 1,000 to 5,000 Angstroms when application
of the photoresist is commenced.
12. A method for applying photoresist to a top surface of a
semiconductor wafer for defining an electronic circuit pattern
thereto, the method comprising:
positioning the wafer on a horizontal turntable with the top
surface facing toward;
discharging a liquid solvent onto a central area of the top
surface;
spinning the wafer to distribute a film of the solvent to a
substantially uniform thickness over the entire top surface of the
wafer;
discharging a liquid photoresist onto the central area of the top
surface of the wafer over the solvent film;
spinning the wafer to distribute the photoresist in a layer over
the entire top surface of the wafer; and
controlling the discharge of photoresist so that the top surface of
the wafer remains entirely wetted by a mobile fluid layer having a
thickness of at least 100 Angstroms of the solvent film during
discharge and distribution of the liquid photoresist.
13. A method according to claim 12 in which the wafer has an
intermediate electronic circuit pattern formed thereon, the
intermediate circuit pattern including at least one of bare silicon
surfaces, high density circuit features and undercut circuit
features; the solvent having a viscosity less than the viscosity of
the liquid photoresist and the solvent film having sufficient fluid
thickness to cause the photoresist to fully cover said bare
silicon, high density and undercut circuit features.
14. A method according to claim 12 in which the solvent film has a
thickness in a range of 500 to 10,000 Angstroms when application of
the photoresist is commenced.
15. A method according to claim 12 in which the wafer is spun at a
first rate of rotation for a first time interval to distribute the
solvent film and then spun at a reduced, second rate of rotation
for a second time interval, and the liquid photoresist discharge is
commenced at the end of the second time interval while the wafer
continues to rotate at said second rate of rotation.
16. A method according to claim 15 in which discharge of the liquid
photoresist is stopped after a third time interval following
commencement thereof and the wafer is spun at an increased, third
rate of rotation for a fourth time interval.
17. A method according to claim 15 in which the first and second
rates of rotation and the first and second time intervals are
arranged so that the top surface of the wafer remains entirely
wetted by the solvent film during discharge and distribution of the
liquid photoresist.
18. A method as in claim 1 in which the liquid solvent comprises a
glycol ether solvent.
19. A method as in claim 1 in which the liquid solvent comprises a
glycol monoethyl ether acetate solvent.
20. A method as in claim 1 in which the liquid solvent comprises an
ethylene glycol monoethyl ether acetate solvent.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to processes for fabricating
integrated circuits, and more particularly to a technique that can
be used in such processes to facilitate application of photoresist
to a wafer without mottling preparatory to a masking and patterning
procedure.
Conventional integrated circuit fabrication processes use one or
more photolithographic steps to define patterns on the surface of a
silicon wafer or similar substrate. These patterns are used in
subsequent steps to form a wide variety of features which,
together, form the active devices and interconnecting circuitry of
an integrated circuit. As shown in FIG. 1, the substrate is
ordinarly a semicircular wafer. A checkerboard pattern of square or
rectangular dice is formed by scribe lines, along which the dice
can be separated once the processing steps that are performed on
the wafer have been completed.
Conventional photolithography comprises a number of well-known
steps. First, the substrate is placed on a turntable. Then, the top
substrate surface, upon which the circuit features are to be
forced, is cleaned by a volatile liquid solvent. The solvent may
include an appropriate bonding agent to assist in adhering a layer
of photoresist, to be applied subsequently, to the substrate
surface. Next, the turntable is operated to spin the wafer as
indicated by the arrow in FIG. 1. This operation spins off the
excess solvent and, conventionally, is continued until the
substrate surface appears dry. Then the turntable is turned off.
After the wafer ceases spinning, a predetermined quantity of liquid
photoresist solution is dispensed onto the top substrate surface.
The turntable is again operated to spin the wafer and thereby
spread the liquid photoresist over the substrate surface. Once the
photoresist layer is dried, it is selectively exposed, using either
a mask or direct writing technique, to pattern each die in
accordance with the desired configuration of one layer of the
overall integrated circuit design layout. Subsequent steps, such as
etching, doping, oxidation and various deposition steps, are
performed using this and subsequently formed and patterned
photoresist layers.
Many different factors can adversely affect the quality, uniformity
and reliability of photolithographic techniques as used in
state-of-the-art integrated circuit fabrication processes. These
factors include the materials, techniques and conditions of
application of the photoresist layer. Ideally, the resultant
photoresist layer will be of such a quality that will enable
accurate photoreproduction of all of the microscopic details of the
mask, first, in the photoresist layer and, second, in the physical
circuit features that are to be formed using this layer.
All of the physical, chemical and environmental factors need to be
controlled carefully. Otherwise, a patterning step can fail, as to
a wafer or an entire batch of wafers, or as to some proportion of
the individual dice, dependent on the statistical effects of the
fault-causing factor. Sometimes, individual factors alone may not
be sufficient to cause problems but, in combination, will adversely
affect the qualities of the photoresist layer and, ultimately, the
quality of the resultant circuit structures.
The particular problem addressed by the present invention is to
avoid mottling. Mottling is a defect in the distribution of a
photoresist layer on a wafer in which portions of a substrate
surface are not coated at all, or are inadequately coated by
photoresist. The factors that lead to mottling are not well
understood. Mottling seems to occur most commonly in photoresist
layers that are applied over steep or undercut topographies, such
as over-etched circuit layers, and on wafers in which the dies are
rectangular rather than square. The affinity of a photoresist
material for the exposed surface materials of the substrate is a
factor. Silicon is phobic to many photoresist compositions, which
is why bonding agents are used, although not with sure success.
Another factor is the structure of the circuit itself. High density
circuits, those having a high density of structural features formed
on the top surface of the substrate, are more prone to problems
than low density circuits.
Some levels of a given fabrication process are likely to be more
sensitive than others. For example, applicant has worked
extensively with Hewlett Packard's CMOSC process. Mottling problems
have often occurred at the lateral channel stop (LCS) level. At
this intermediate level, the surface structure includes substantial
bare silicon and an etched nitride layer which presents an undercut
lip to the photoresist. The occurrence of mottling was widely
variable, and highly sensitive to minor changes of conditions or
combinations of conditions. When all conditions were apparently
optimal, mottling would affect as few a 1% of dice. When some
conditions deteriorated, mottling would increase and commonly
affect 30% of the dice, and on occasion entire lots of wafer have
had to be reworked.
Prior attempts to discern and control the source of mottling have
met with little success. One approach has been to simply flood the
wafer surface with photoresist, using two or more times the usual
amount of liquid photoresist. This met with little success and is
wasteful of photoresist. It is also susceptible to problems, such
as filter clogging, that affect the accurate metering of higher
volumes of photoresist. It is also possible to allow the
photoresist to stand on the substrate surface for an extended
period of time before spinning off the excess, to give the
photoresist time to fully wet the substrate surface. This approach
can result in radially uneven distribution of the photoresist
layer. Radially uneven photoresist distribution can interfere with
close contact between the mask and the photoresist surface over the
entire wafer, and uneven exposure of the photoresist layer,
resulting in blurring of exposed boundaries in many dice.
Accordingly, a need remains for a technique for more reliably
applying photoresist to wafers without mottling.
SUMMARY OF THE INVENTION
One object of the invention is to improve the application of
photoresist to a top surface of a substrate in the fabrication of
integrated circuits.
A second object is to improve the yield and consistency of yield of
operative dice, particularly of those containing high density
circuitry, circuit structures with substantial variations in
topography, or resist phobic exposed surface materials.
A particular object of the invention is to reduce, and preferably
eliminate, mottling in the application of photoresist layers to
substrates as aforesaid.
An additional object is to reduce the sensitivity of successful
photoresist application to minor changes in conditions.
The invention is an improvement in existing techniques for applying
photoresist to integrated circuit wafer surfaces, which
substantially reduces, and usually eliminates, mottling in
fabrication process steps and integrated circuit designs where
mottling would otherwise occur. Briefly, the improved technique
calls for dispensing the liquid photoresist onto the top surface of
the substrate while it is still wet with cleaning solvent.
Preferably, this is done while the substrate is still spinning in
connection with distribution of the cleaning solvent.
More particularly, in a preferred embodiment, the method comprises
positioning the wafer on a horizontal turntable with the top
surface facing upward; discharging a liquid solvent onto a central
area of the top surface; spinning the wafer to distribute a film of
the solvent to a substantially uniform thickness over the entire
top surface of the wafer; discharging a liquid photoresist onto the
central area of the top surface of the wafer over the solvent film;
and spinning the wafer to distribute the photoresist over the
entire top surface of the wafer. The discharge of photoresist is
controlled so that the top surface of the wafer remains entirely
wetted by the solvent film during discharge and distribution of the
liquid photoresist. The wafer may have an intermediate electronic
circuit pattern formed thereon, which includes bare silicon
surfaces, high density circuit features or undercut circuit
features. The solvent used should have a viscosity less than the
viscosity of the liquid photoresist and the solvent film should
have sufficient fluid thickness to cause the photoresist to fully
cover said bare silicon, high density or undercut circuit features.
The solvent film should have a thickness of at least 100 Angstroms
when application of the photoresist is commenced, and preferably
has a thickness in a range of 500 to 10,000 Angstroms when
application of the photoresist is commenced.
The foregoing and other objects, features and advantages of the
invention will become more readily apparent from the following
detailed description which proceeds with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a top plan view of a silicon wafer, schematically showing
dice inscribed in the substrate surface and mottling in a layer of
photoresist applied over the substrate surface.
FIG. 2 is an enlarged cross-sectional view taken along lines 2--2
in FIG. 1, showing an exposed silicon surface, an undercut nitride
layer, and a boundary between the layer of photoresist and a
mottled region.
FIGS. 3 and 4 are cross-sectional views similar to FIG. 2, showing
application of a film of liquid solvent to wet the substrate
surface followed by application of a layer of liquid photoresist
over the liquid solvent film in accordance with the invention.
FIG. 5 is a time-based graph of the technique of the present
invention for discharging and spreading liquid photoresist on the
substrate surface while the surface is still wet with solvent.
DETAILED DESCRIPTION
Referring to FIG. 1, a typical form of substrate is a silicon wafer
10, although the invention is applicable to other substrate
materials. The substrate has scribe lines 12 which define square or
rectangular dice, such as die 14 and die 18. When photoresist is
applied in accordance with conventional procedures, mottling of the
photoresist layer may result, as discussed above, leaving regions
that are not adequately covered by photoresist. These regions are
indicated in FIG. 1 by photoresist boundaries 16. Absent mottling,
die 14 will be adequately covered with photoresist and can be
appropriately patterned to make an operative integrated circuit.
Die 18, on the other hand, has a portion that is not covered by
photoresist and, therefore, cannot be patterned to form circuit
features that are critical to making the device operative. The
mottling features tend to display characteristics that are
influenced by the spinning of the wafer to spread photoresist, such
as a radial orientation, and characteristics of the dice as laid
out on the wafer, such as boundaries along scribed lines, areas of
bare silicon or high density circuit features, which are typically
rectilinear in shape.
FIG. 2 shows the wafer 10 with a substrate surface 22 and scribe
line 12 inscribed in the bare silicon surface 22 between dice 14,
18. For purposes of illustration, a good die 14 is shown alongside
a defective die 18. Both dice have an earlier formed circuit
feature 24, which could be, for example, a layer of nitride that
has been patterned in a prior processing step, such as the lateral
channel stop (LCS) level mentioned above. Layer 24 is shown with an
undercut perimeter 28. Following conventional photoresist
application procedures, a quantity of liquid solvent containing a
bonding agent has been applied and spun off until apparently dry.
In the example mentioned above, the liquid composition was ethylene
glycol monoethyl ether acetate, also known as cellusolve acetate or
EGMEA (CH.sub.3 COOCH.sub.2 CH.sub.2 OC.sub.2 H.sub.5) as the
solvent, and 30% hexamethyldisilazane, or HMDS (C.sub.6 H.sub.19
Si.sub.2 N), as the bonding agent (and which is also a solvent for
photoresist). Spinoff leaves a thin residual film 26 of bonding
agent and solvent on the exposed surfaces of the substrate and
structures 24. The surface visually appears dry, meaning that the
film 26 has thickness less than 500 Angstroms. The solvent and
bonding agent are volatile, but bond to some extent with the
substrate surface, so the resultant layer 26 may actually be as
thin as a monolayer or a few molecular layers, probably under 50
Angstroms and very likely under 100 Angstroms thick.
The conventionally-applied photoresist layer is shown terminating
in a boundary 16 which, for illustrative purposes, is located at
scribe line 12 on the bare silicon surface. This boundary can occur
at locations other than scribe lines, for example, along device
features, such as undercut 28 in layer 24. The photoresist boundary
is shown with a bulbous shape in FIG. 2 because liquid photoresist
has a high viscosity and surface tension. Although mottling may
take a number of different forms that vary from what is shown in
FIG. 2, this form and other forms are effectively curtailed by the
invention, as next described.
FIG. 3 shows a wafer having the same initial conformation as that
of FIG. 2, namely a substrate 10 with scribe lines 12 in surface 22
and deposited and etched layer structures 24 in each die. The first
step of the technique of the invention is to apply a film 30 of
liquid solvent (such as EGMEA with HMDS) over the entire substrate
surface and to distribute the solvent approximately uniformly
through a predetermined range of thickness indicated by reference
numeral 32. This range of thickness can vary but must be sufficient
to wet the substrate surface with liquid solvent.
FIG. 5 is a graph illustrating the technique of the invention along
a time line. The sequence commences with the discharge of solvent
onto the surface of the substrate. The solvent accumulates on the
substrate to an initial thickness shown along the vertical axis.
Next, the turntable is operated to spin off excess solvent. This
procedure reduces the thickness of the solvent, as indicated by the
descending portion of the curve in FIG. 5. If the wafer were spun
until the solvent appeared completely dried, as in the prior art,
the solvent thickness would ultimately reduce to the thickness of
the residual bonding layer, as indicated at the far right in FIG.
5, and as shown as layer 26 in FIG. 2. This is not allowed to
happen in the present invention.
Referring to FIG. 4, a predetermined quantity of liquid photoresist
solution 36 is dispensed onto the wafer, while it is still spinning
from the step of FIG. 3, over the liquid solvent film 30. The
liquid photoresist solution is immediately spread, as it is being
discharged, over the entire surface of the wafer. As shown in FIG.
5, this is done at a time when the liquid solvent still has a
measurable fluid thickness, well before it has dried down to just a
residual bonding layer of 100 Angstroms or less.
Discharging and spreading the liquid photoresist while the
substrate surface remains wet with a fluid film of solvent
facilitates distribution of the photoresist over the entire surface
of the wafer, over bare silicon surface areas, scribe lines and
etched layers deposited on the substrate surface, including those
with undercut boundaries. Incidence of mottling, even at heretofore
highly sensitive levels of fabrication, is reduced to nil.
Sensitivity of successful photoresist application to minor
variations in multiple conditions is greatly reduced.
While the precise mechanisms are not fully understood, it is
believed that the liquid solvent film 30 facilitates spreading the
photoresist solution 36 in several ways. First, the fluid solvent,
apparently more philic to silicon than liquid photoresist,
effectively pre-wets the substrate and device feature surfaces.
This enables the photoresist more readily to come into intimate
contact with such surfaces. Second, the photoresist solution
already contains some proportion of the solvent, and is readily
miscible with it. The solvent film 30 may locally dilute the
photoresist solution at the interface between the photoresist
solution and the liquid film, thereby reducing the viscosity of the
photoresist at such interface. This dilution, and possibly an
active solution-type interaction, may also facilitate the migration
of photoresist into undercut areas. The liquid solvent film may
also reduce the overall surface tension of the liquid photoresist
layer.
Regardless of the precise mechanism or combination of mechanisms,
the technique of the present invention reliably and uniformly coats
substrates with photoresist, even in high density and high
topography circuit designs that are particularly prone to mottling.
It does this without flooding the wafer surface with liquid
photoresist. Conventional quantities of photoresist can be used
with virtually 100% reliability.
As mentioned above, a predetermined measurable amount of liquid
solvent must be left on the substrate surface when the liquid
photoresist is discharged. Simply put, the substrate surface must
be wet with fluid solvent, not apparently dry, though an excess of
solvent is not desirable. A suitable amount, or thickness, of
solvent film may be determined a number of different ways. One is
empirically, by testing different time intervals. If too much
solution remains, the photoresist may not consistently coat the
substrate surface, affecting repeatability. If too little remains,
mottling will begin to occur. Another is visually: When first
dispensed onto a stationary wafer, the solvent appears colorless
and transparent. During spinoff, as the thickness of the liquid
film approaches the wavelength of light (beginning at about 15,000
Angstroms thickness), under visible light (center wavelength about
5,500 Angstroms, bandwidth about 2,000 Angstroms) interference
colors will appear (alternating between red and green shades).
While these colors are still apparent, the photoresist is
dispensed. If the photoresist were not dispensed during this time
interval and spinoff were continued in conventional manner, the
interference colors would disappear (at about 500 Angstroms film
thickness) and the wafer surface would soon turn silver or grey,
proceeding from the perimeter toward the center of the wafer.
Experimentally, mottling was found to begin to appear at this
stage.
EXAMPLE
Following is an implementation of the invention for applying
photoresist in the lateral channel stop level of the Hewlett
Packard CMOSC process. With little or no change, it can be applied
to any level in any fabrication process on silicon, although it may
only be needed at one level. Its principles can be readily extended
to other materials, such as GaAs. This implementation is preferably
computer-controlled for consistency, but has been successfully
executed manually.
1. Position wafer on-axis in chuck (turntable). Conventional
solvent and photoresist dispensers are positioned centrally over
the chuck.
2. Turn on liquid solvent dispenser to discharge solvent (EGMEA
with 30% HMDS; viscosity: 0.5-5.0 centistokes).
3. Wait 2 seconds, then turn off solvent, discharging about 3.5
milliliters of solvent onto wafer top surface.
4. Wait 5 seconds.
5. Spin wafer at 4000 r.p.m. for 2.6 seconds, which spins off the
majority of the solvent.
6. Slow the chuck to 500 r.p.m. and wait 1 second. At this stage,
the solvent film thickness shows interference colors consistent
with a thickness in the range of 1,000 to 5,000 Angstroms.
7. Turn on photoresist dispenser to discharge liquid photoresist
(Shipley MICROPOSIT 1400-27, containing 62% EGMEA, 6% n-butyl
acetate, 6% Xylene; viscosity: 17.5-17.9 centistokes) onto wafer
atop the liquid solvent film.
8. Wait 5 seconds while continuing to spin wafer at 500 r.p.m. to
spread photoresist over wafer surface.
9. Turn off photoresist dispenser. This discharges about 3.5
milliliters of photoresist.
10. Spin wafer at 3400 r.p.m. for 12 seconds.
11. Turn off chuck. This leaves a photoresist layer of about 14,000
Angstroms thickness.
12. The wafer is released from the chuck and transferred to an oven
for baking the photoresist layer in conventional manner.
Having illustrated and described the principles of the invention in
a preferred embodiment, it should be apparent to those skilled in
the art that the invention may be modified in arrangement and
detail without departing from such principles. I/we claim all
modifications and variations coming within the scope and spirit of
the following claims.
* * * * *